I am very in favor of mandatory service as a strategy to improve nations but I do want to point out that some of the most impressive ones (South Korea and Israel come to mind) are extremely homogenous.
edited: punctuation
- Enemy nations will be somewhat less motivated to invade if they know they will be facing a population that is entirely armed and ready to resist.
- Putting the population into the service of the country helps build a sense of national cohesion by giving a shared sense of mission and duty.
- Training the citizens to fight could also be a tool to help prevent dictatorships where the military persecutes the people; a military of citizen-soldiers would be less likely to see their own people as a "them" and more as an "us."
I work with some people in Armenia (which has mandatory conscription) - and from what I've observed that military service made the Us v. Them between Armenians and Azerbaijani stronger than perhaps it would have been without said military service (a drunken encounter between one of said Armenians and an Azerbaijani person outside of Armenia literally came to blows for no real reason I could observe except Us v. Them (well, and alcohol)).